1) Field of the Invention
The present invention relates to an operational amplifier circuit, and in particular to an operational amplifier circuit suitably used for a MOS integrated circuit.
2) Description of the Related Art
MOS operational amplifiers have been widely used for adding and subtracting analog signals and so forth. For example, a prior art reference "MOS Operational Amplifier Design A Tutorial Overview", IEEE Journal of Solid-State Circuits, VOL. SC-17, No. 6, December 1982, Pages 969 to 981, discloses a conventional MOS operational amplifier circuit. A typical MOS operational amplifier circuit is composed of a differential amplifier circuit, a level shift circuit and an output circuit.
This operational amplifier circuit serves as an AB operational amplifier which applies a large current to a load connected to its output when a signal is received, but decreases a current running through the output stage when no signal is received.
However, the conventional MOS operational amplifier circuit sometimes outputs an output signal having distortion. Namely, the NMOS FET of the output circuit is completely turned on and is completely turned off responding to the internal signal generated by the level shift circuit. The distortion is created in the output signal when either NMOS FET or PMOS FET is completely turned off.
Further, the conventional MOS operational amplifier circuit sometimes applies a large current to a small output load resistor so as to generate an output signal at a power source voltage level. Accordingly, it raises a problem in that large scale integration with a high density is difficult since the ratio W/L (where W is gate width and L is gate length) of the MOS FETs which constitute the output stage should be large.